OS 10.6 - Lacie 1TB external problem.

I recently installed Linux Mint 7 from my external HD and everything went smooth. I went to erase the image from my HD and I'm getting an error in disk utility that says something along the lines of "couldn't unmount image". So basically I'm stuck with a useless external HD... I already booted into the Snow Leopard install disk and tried to repair, erase, and restore the HD but nothing is working. Can someone help me resolve this issue?